1. Field of the Invention
The invention involves an aqueous developer and a process for preparing flexographic printing forms by imagewise exposure of a recording material, washing off the uncrosslinked portions of the layer with a special aqueous developer, and drying the resulting treated recording material.
2. Description of Related Art
The use of photopolymerizable plates for preparing flexographic printing forms is known. The printing surface is produced by imagewise exposure of a layer that is photopolymerizable by actinic radiation and by subsequently removing the unexposed, unphotopolymerized areas of the printing plate. Aqueous solutions or organic solvents are usually employed as the developer, depending on the binder.
More and more aqueous developers have been used in recent years due to their lower environmental impact and decreased health hazard. Examples of aqueous developers being used are aqueous solutions of alkali metal hydroxides or carbonates and aqueous solutions of ionic or nonionic surfactants. Developers containing surfactants are described in JP6-297829.
The washoff process is conducted mostly in washoff devices with rotating brushes of various materials. Developers containing a surfactant frequently have a low washoff rate.
Therefore, the problem involved in the present invention was to improve the washoff process for aqueous-developable flexographic printing plates, in particular, to increase washoff rate.
This problem is solved by an aqueous developer for photopolymerizable flexographic printing plates containing a) a combination of at least one amino alcohol and/or at least one amino carboxylic acid with at least one surfactant or b) a combination of at least two surfactants, and by a process for preparing flexographic printing forms by using this developer.
The developer of the present invention led surprisingly, to washoff rates significantly higher than with conventional developers containing only a surfactant. The advantage of the developer of the present invention was all the more surprising at JP6-297829 discloses that amino alcohols are supposed to have no effect on the washoff rate.
The present aqueous developer contains, as components essential to the invention, at least one amino alcohol or at least one amino carboxylic acid in combination with a surfactant. Suitable amino alcohols can contain one or more amino groups and also one or more hydroxy groups. Suitable amino carboxylic acids can contain one or more amino groups and also one or more carboxyl groups. Preferred amino alcohols and amino carboxylic acids have the formula R1xR2yR3zN, wherein R1=H; R2=-CnH2n+1xe2x88x92m-(OH)m for the amino alcohols or R2=xe2x80x94CnH2n+1xe2x88x92mxe2x80x94(COOH)m for the amino carboxylic acids, wherein R3=alkyl, aryl, substituted alkyl, or substituted aryl groups; x=0-3; y=1-3; z=0-2; n=1-15; m=nxe2x88x921; and X+y+z=3. Examples of suitable amino alcohols are 1-amino-2-propanol, 2-amino-2-ethyl-1,3-propanediol, N-butyl-diethanol, triethanolamine, 3-diethylamino-1,2-propanediol, 2-amino-2-methyl-1-propanol, 2(N-benzyl-N-methylamino)ethanol, 2-amino-1-phenyl-1,3-propanediol, 2-(2-aminoethylamino)ethanol, diisopropylamino-1,2-propanediol, and tris(hydroxymethyl)aminomethane. Triethanolamine, 3-diethylamino-1,2-propanediol, and tris(hydroxymethyl)aminomethane are particularly preferred. Preferred amino carboxylic acids are nitrilotriacetic acid, serine, aspartic acid, and polyaspartic acid. The quantity of amino alcohols or amino carboxylic acids is preferably 0.05 to 2.0 percent by weight, particularly 0.1 to 1.0 percent by weight.
The quantity of surfactant is 0.1 to 5 percent by weight, preferably 0.1 to 2 percent by weight. All known nonionic and ionic (anionic, cationic, and amphoteric) surfactants can be used in the sense of the invention, combined with amino alcohols or amino carboxylic acids as well as with each other. Examples of suitable surfactants are sodium dodecylbenzene sulfonate, sodium lauryl sulfate, sodium 2-ethylhexylsulfate, ethylenediamine triacetate-coconut alkylacetamide trisodium salt, diisooctyl sulfosuccinate sodium salt, tricarboxylmethyl diaminoalkylamide, lanolin polyglycol ether, sodium lauryl sulfate, phenol polyglycol ether, amphoteric glycine derivatives, and polyoxyethylene glycerol oleoricinolates. Ethylenediamine triacetate-coconut alkylacetamide trisodium salt, diisooctyl sulfosuccinate sodium salt, sodium lauryl sulfate, and polyoxyethylene glycerol oleoricinolates are particularly preferred. The present aqueous developer can also comprise a combination of at least two surfactants as described above in aqueous solution.
The present developer is prepared by dissolving the appropriate quantities of the components to be used in water before the development process. The developer of the present invention can be used in all conventional washoff devices, preferably in high-quality steel devices. All current brush materials, such as, for example, nylon, polyester, or polypropylene, can be used. The washoff process is conducted preferably at an elevated temperature, usually at xe2x89xa730xc2x0 C., particularly at 50xc2x0 C.
The present developer is suitable for developing aqueous-developable recording materials having photopolymerizable layers. These layers contain preferably hydrophilic polymers optional hydrophobic polymers, photopolymerizable monomers, photoinitiators, and added auxiliaries, such as plasiticizers, fillers, stabilizers, dyes, inhibitors, etc.
Aqueous-developable printing plates contain, as an essential binder, polymers having hydrophilic groups, such as, for example, hydroxyl, carboxyl, amino, nitrile, sulfate, and phosphate groups, or combinations of these groups. Examples are polyvinyl alcohols, polyvinyl acetates, copolymers of (meth)acrylic acid and (meth)acrylates, cellulose derivatives, polyesters, polyamides, carboxylated butadiene/acrylonitrile polymers, core-shell polymers having hydrophilic groups in the shell polymers, or block copolymers having hydrophobic and hydrophilic groups. Also useful are mixtures of these various polymers or mixtures with hydrophobic binders, such as thermoplastic elastomer copolymers, particularly with thermoplastic elastomeric block copolymers. Linear and radial polystyrene/polybutadiene/polystyrene or polystyrene/polyisoprene/polystyrene block copolymers are preferred. Aqueous-developable flexographic printing plates are described in U.S. Pat. No. 5,175,076; European Publication EP-A 0 784 232; U.S. Pat. Nos. 4,621,044; 4,177,074; and 5,075,192.
The photopolymerizable materials can be applied onto commercial supports and exposed imagewise by current methods. After being developed with the developer of the present invention, the printing forms are dried, followed optionally by a chemical posttreatment and/or postexposure.